1. Field of the Invention
The present invention relates to an electrodeionization apparatus. More specifically, the present invention relates to an electrodeionization apparatus having excellent desalting capacity and operational stability even with a high loading of weak ions including CO2 and silica.
2. Description of the Related Art
Conventionally, the deionized water used in various industries including semiconductor manufacturing industry, liquid-crystal manufacturing industry, pharmaceutical industry, food industry and power industry, etc., as well as in research facilities, is frequently produced by using a type of electrodeionization apparatus described in, for example, Japanese Patent No. 1782943 (JP 1782943), JP 2751090 and JP 2699256. In the electrodeionization apparatus, multiple anion exchange membranes and cation exchange membranes are alternately arranged between a cathode and an anode to alternately form concentrating compartments and desalting compartments. The desalting compartments are filled with an ion exchanger that is constituted of a mixed ion exchange resin of anion exchange resin and cation exchange resin, ion exchange fibers or the like. Moreover, to reduce the electrical resistance of the concentrating compartments and thereby maintain the required current magnitude, an electrodeionization apparatus including concentrating compartments filled with an ion exchanger has also been proposed, as described in Japanese Patent Application Laid Open No. 2002-205069.
In such an electrodeionization apparatus, each kind of ion conducted into the desalting compartments reacts with the ion exchanger and moves in the ion exchanger along the direction of electrical potential gradient in a specific manner according to its affinity, concentration and mobility. The ions further traverse the membranes to maintain all compartments electrically neutral. Moreover, the ions are removed from the desalting compartments and concentrated in the adjacent concentrating compartments because of the semipermeable property of the membranes and the directionality of electrical potential gradient. That is, the cations and the anions permeate through the cation exchange membranes and the anion exchange membranes, respectively, and are concentrated in the concentrating compartments. Therefore, the water produced from the desalting compartments can be recovered as deionized water (pure water).
The above electrodeionization apparatus is capable of efficiently implementing a desalting treatment without the requirement of regenerating the ion exchange resin. Therefore, the electrodeionization apparatus has the capability of continuously producing deionized water of extremely high purity.
The cited Patent documents include:
Patent document 1: JP 1782943
Patent document 2: JP 2751090
Patent document 3: JP 2699256
Patent document 4: Japanese Patent Application Laid Open No. 2002-205069
However, when the loading of weak ions including CO2 and silica in the electrodeionization apparatus is high, i.e., when the concentration of the weak ions including CO2 and silica in the water being treated is high or the amount of such water being treated is large, the quality of the deionized water produced is deteriorated as indicated by the specific resistivity thereof. Moreover, the electrical resistance of the system gets higher after long-term use, so that the operational stability of the apparatus is lowered.
Moreover, Japanese Patent Application Laid Open No. 2002-205069 disclosed that the concentrating compartments can also be filled with an ion exchanger. As described in the prior art document, an electric conductor like an ion exchanger is filled in the concentrating compartments merely for maintaining the required current magnitude, so that the ratio of anion exchanger to cation exchanger of the ion exchanger is not particularly discussed. Therefore, as in the desalting compartments, a mixed ion exchange resin having the same “volume ratio of anion exchange resin to cation exchange resin” of 7:3 is filled in the concentrating compartments in the examples of Japanese Patent Application Laid Open No. 2002-205069.
To solve the problems of the prior art, one object of this invention is to provide an electrodeionization apparatus that has excellent desalting capacity and operational stability even when the loading of weak ions including CO2 and silica is high.